A predetermined bearing pre-pressure is applied to the wheel bearing apparatus in order to assure a desirable bearing rigidity. In the wheel bearing apparatus of a so-called third generation type, that includes a wheel hub directly formed with an inner raceway surface on its outer circumference and an inner ring press-fit onto the wheel hub, the control of the amount of bearing pre-pressure has been performed by accurately controlling the abutting surfaces between the wheel hub and the inner ring and by setting the fastening torque (axial force) of a securing nut, when fastening together the wheel hub and a constant velocity universal joint.
The amount of bearing pre-pressure not only influences the bearing life and bearing rigidity, but also has a strong relationship to the running of a vehicle and improved fuel consumption. Thus, the amount of bearing pre-pressure is proportional to the rotational torque. Accordingly, a reduction in the amount of bearing pre-pressure can reduce the rotational torque. Thus, this contributes to improved fuel consumption. On the contrary, the bearing inclination angle is a main cause of the bearing rigidity which is inversely proportional to the bearing pre-pressure. Thus, an increase in the amount of pre-pressure can improve the bearing rigidity. This reduces the bearing inclination angle as well as suppresses the inclination of a brake rotor during running on a curved road. Accordingly, it is possible to provide a wheel bearing apparatus, that is excellent not only in the bearing life but also in fuel consumption, by setting an optimum pre-pressure of the bearing.
A method is known, as shown in FIG. 5(a), for controlling the pre-pressure (negative clearance) of bearings. This pre-pressure monitoring apparatus 51 is adapted to perform the pre-pressure setting by measuring a torque while rotating a bearing when the wheel bearing apparatus is assembled. The pre-pressure monitoring apparatus 51 includes a gear 53, with a rubber roller, adapted to contact with an outer circumference of the outer member 52 at the inner-side of its body mounting flange 52b. A driving gear 54 meshes with the gear 53. An electric motor 55 rotates the driving gear 54. A torque detector 56, including a power meter, detects the driving torque of the motor 55. A comparator 57 compares the detected rotational torque with a predetermined value.
In the pre-pressure monitoring apparatus 51, the motor 55 is driven to rotate the outer member 52 via the gears 54, 53. The rotational torque of the outer member 52 is detected by the torque detector 56. The amount of pre-pressure is measured based upon the detected rotational torque. When the measured amount of pre-pressure has reached a predetermined value, an amount of pre-pressure suitable for the wheel bearing apparatus 50, a swing motion type caulking apparatus 58 is retracted. Even after the caulking operation has been completed by the caulking apparatus 58, the rotational torque is monitored. The rotational torque is confirmed so that the amount of pre-pressure is proper.
FIG. 5(b) is a graph showing variations of the position A of a caulking die 58a of the swing motion type caulking apparatus 58. The rotational torque T (ordinate) is relative to a time duration “t” of caulking (abscissa). The caulking operation is started by gradually descending the position A of the caulking die 58a. Application of pre-pressure to the wheel bearing apparatus is begun at a point of time “t0”. Variation of the rotational torque T is also begun at this time. When a magnitude of the variation has reached a predetermined value Δ at a point of time “t1”, it is determined that an optimum pre-pressure has been applied to the wheel bearing apparatus 50. The caulking operation is then stopped. Then, the position A of the caulking die 58a is returned to the origin. The pre-pressure monitoring apparatus 51 can perform setting an amount of pre-pressure while measuring the rotational torque by rotating bearings on an assembly of the wheel bearing apparatus 50 (See, JP 11-44319 A).
As described above, the prior art technology is structured so that an amount of pre-pressure on the bearings can be accurately and stably controlled by detecting rotational torque of the bearings by the pre-pressure monitoring apparatus 51, measuring the amount of pre-pressure based on the detected rotational torque, and operating the swing motion type caulking apparatus 58. Operation of the caulking apparatus is by determining whether the measured amount of pre-pressure is optimum or not. In manufacturing the wheel bearing apparatus, with the self-retaining structure, it is possible to accurately and stably control the amount of pre-pressure of the bearings, via the rotational torque, if each dimension of the groove diameters of the outer member 52, the wheel hub 61 and the inner ring 62 press-fit on the wheel hub 61, is finished within a desirable range. Thus, it is difficult to accurately control the amount of pre-pressure if there are variations in the dimensions of the groove diameters. In addition, since the detection of rotational torque is performed during manufacture of the wheel bearing apparatus, by the swing motion type caulking apparatus 58, it is impossible to re-assemble the wheel bearing apparatus by replacing part of its structural elements.